Hydroprocessing includes processes which convert hydrocarbons in the presence of hydroprocessing catalyst and hydrogen to more valuable products. Hydrocracking is a hydroprocessing process in which hydrocarbons crack in the presence of hydrogen and hydrocracking catalyst to lower molecular weight hydrocarbons. Depending on the desired output, a hydrocracking unit may contain one or more beds of the same or different catalyst. Slurry hydrocracking is a slurried catalytic process used to crack residue feeds to gas oils and fuels.
Hydrotreating is another hydroprocessing process used to remove heteroatoms such as sulfur and nitrogen from hydrocarbon streams to meet fuel specifications and to saturate olefinic compounds. Hydrotreating can be performed at high or low pressures, but is typically operated at lower pressure than hydrocracking. Due to environmental concerns and newly enacted rules and regulations, saleable fuels must meet lower and lower limits on contaminates, such as sulfur and nitrogen. New regulations require essentially complete removal of sulfur from diesel. For example, the ultra-low sulfur diesel (ULSD) requirement is typically less than about 10 wppm sulfur.
A typical refinery configuration comprises multiple separate hydrotreatment units for sulfur removal from different streams such as naphtha, distillate, and heavy residual oil (where required). Hydrocracking is often included in the configurations to upgrade heavier, low value products into distillate and lighter products. Often a naphtha hydrotreating unit is a low cost unit requiring relatively low pressure conditions. In contrast, distillate hydrotreating unit requires higher pressure, while a hydrotreating or hydrocracking unit for a heavy residual oil would require even higher pressures. The separation of the reactor sections for the different hydrotreating units is often the optimal economic solution based on equipment operational cost savings (e.g., processing each stream at minimum pressure).
All hydroprocessing units typically utilize a relatively low pressure (i.e., between 100-150 psig) stripping of hydrogen sulfide from the hydroprocessed effluent. Optimum stripping of naphtha involves re-boiling of the naphtha either with a fired heater or a heat exchanger to remove LPG (C4-hydrocarbons). Optimum stripping of heavier oils to remove naphtha and lighter typically uses steam stripping to minimize capital and operating costs (e.g., by elimination of the heater) and to prevent coking from high pressure re-boiling or off gas compression requirements for low pressure re-boiled stripping. In addition, simple combination of two effluent streams would require rigorous fractionation of naphtha from diesel or heavier oils in addition to stripping which is both capital and energy intensive. Thus, due to the different stripping processes and product requirements, the two stripping sections cannot merely be combined.
However, it is believed that a process for stripping multiple effluent streams is desirable because if the stripping sections of a naphtha hydrotreating unit and a diesel hydrotreating unit were combined, less equipment and plot space would be utilized. Therefore, the present invention seeks to address the problems associated with combining the stripping sections for two different hydroprocessing units.